Forbid logins on the SMTP-STARTTLS port

Also, introduce the outline of a framework to handle message sending
differently to message receipt.

internal/smtp/receiver.go

@@ -0,0 +1,20 @@
+package smtp
+
+import (
+	"fmt"
+	"io"
+)
+
+type Receiver struct{}
+
+func (r *Receiver) Name() string {
+	return "smtp-starttls"
+}
+
+// It seems odd to have a Receiver called Send, but what we're looking for is an
+// attempt from an arbitrary source to send an email to known accounts.
+//
+// We might want to clean this interface
+func (r *Receiver) Send(from string, to []string, reader io.Reader) error {
+	return fmt.Errorf("Not yet implemented")
+}